The role of local peridontal ligament (PDL) cells, peripheral monocytes, and in situ macrophages in the origin and histogenesis of osteoclasts along a previously resorbing alveolar bone surface will be investigated in 6-8 week old rat PDL. The nuclei of circulating monocytes will be selectively labeled with 3H-thymidine (3H-Tdr), and the cytoplasm of the PDL macrophages labeled by intravenous injection of a carbo particle suspension. Computerized histomorphometry of autoradiographs of 3 microns serial sections will categorize all cells according to cell type, geographic location (zone), nuclear volume, autoradiographic grain count, and presence or absence of carbon particles. Dynamics of these cellular parameters will be studied during stimulation of osteoclast histogenesis with parathyroid extract (PTE) or biomechanical induction (accelerated approximal drift). A kinetic model for osteoclast histogenesis will be derived. An important aspect of this study is the development of a digitizer-minicomputer-plotter system for automated quantitation and data processing of cell kinetic parameters of bone histomorphometry. This will open new horizons for investigating the interaction of DNA synthesis, mitosis, change in nuclear volume, influx of hematogenous cells, and cell migration patterns associated with bone cell histogenesis. In conjunction with tetracycline labeled iliac crest bone biopsies incubated in 3H-Tdr, the proposed computerized methods could evolve into a powerful clinical tool for diagnosing and evaluating treatment of skeletal disease. A more direct dental application would be as an in vivo PDL model for investigating the biological response of biomechanically defined force systems.